The present disclosure relates to a fixing device that fixes a toner image onto a paper sheet and an image forming apparatus including the fixing device.
Electrophotographic image forming apparatuses such as copying machines and printers are conventionally provided with a fixing device that fixes a toner image onto a paper sheet. As a fixing method used in such a fixing device, a “heat roller method” may be used in which a toner image is fixed onto a paper sheet at a fixing nip formed between a pair of rollers. The heat roller method is superior in terms of the thermal efficiency or the safety. Meanwhile, due to a demand for reduction of the warm-up period and energy saving, a “belt method” is also known in which a fixing nip is formed using a fixing belt to be heated by a heat source.
The belt method includes a method in which a fixing belt is caused to slide on a pushing member in contact with the inner circumferential surface of the fixing belt (hereinafter, referred to as “slide belt method”). According to such a slide belt method, it is possible to reduce the heat capacity of members in contact with the fixing belt and perform more focused heating on the fixing belt compared to the case where a fixing belt is provided around a roller.
However, employment of the above-described slide belt method involves a problem in that stress is concentrated on the fixing belt in a neighboring region of an end portion of the fixing nip, and therefore the fixing belt is locally deformed. In particular, the fixing belt may be broken earlier when the fixing belt is formed of a relatively hard material (for example, a metal) and the stress concentration is intense. The early breaking of the fixing belt makes the fixing device unusable and therefore shortens the life of the fixing device. For this problem, a configuration is known in which the cross-sectional shape of a central portion of the pushing member in contact with the inner circumferential surface of the fixing belt is made different from the cross-sectional shapes of opposite end portions thereof to avoid the deformation of the fixing belt.
Incidentally, a fixing device fixes toner images onto paper sheets in a variety of sizes ranging from small to large. The case where the fixing device fixes a toner image onto a paper sheet in a first size (for example, B5 paper sheet) will be discussed. In this case, heat is consumed by heating of the paper sheet in the first size in a region of the fixing belt through which the paper sheet in the first size passes (hereinafter referred to as “first size sheet passage region”). On the other hand, no heat is consumed in regions which are outside the first size sheet passage region and through which a paper sheet in a second size (for example, A4 paper sheet) having a larger width than the paper sheet in the first size passes (hereinafter, referred to as “non-first size sheet passage regions”). Thus, the temperature of the non-first size sheet passage regions will be higher than the temperature of the first size sheet passage region, and the distribution of the temperature of the roller and the belt forming the fixing nip will be non-uniform. For this problem, a configuration is known in which the temperature of the non-first size sheet passage regions is prevented from rising to be excessively higher than the temperature of the first size sheet passage region with a thermally conductive member.